Press hardened steels are typically high strength and have been used in automotive applications for reducing weight while improving safety performance. Hot stamped parts have mainly been made from either bare steel, which must have the oxide removed after stamping, or from steel with an aluminized coating. The aluminized coating provides a barrier form of corrosion protection. A zinc-based coating further provides hot stamped parts with active, or cathodic corrosion protection. For instance, hot dip galvanized steel typically includes a Zn—Al coating and hot dip galvannealed steel typically includes a Zn—Fe—Al coating. Due to the melting temperature of zinc, liquid zinc can be present during the hot stamping process and lead to cracking due to liquid metal embrittlement (LME). Time at the high temperature required for austenitization of the steel substrate prior to hot stamping allows for diffusion of iron into the galvannealed coating to avoid LME. However, during the time required to allow for sufficient iron diffusion, zinc in the coating can be lost due to vaporization and oxidation. This oxide may also exhibit poor adhesion and tend to flake off during stamping.
Disclosed herein is a pre-alloying heat treatment performed after galvannealing and prior to the hot stamping austenitization step. The pre-alloying allows for shorter time at the austenitization temperature to form a desired α-Fe phase in the coating by increasing the concentration of iron. This also decreases the loss of zinc, and a more adherent oxide exists after hot stamping.